1. Field of the Invention
The present invention relates to an image forming apparatus in which a cleaning means cleans off toner remaining on an image carrier after a visible image has been transferred from the image carrier onto a recording medium.
2. Description of the Related Art
In an image forming apparatus of the electrophotographic type, such as a laser beam printer, there is provided a developing device that effects conversion of an electrostatic latent image into a visible image by supplying toner onto the electrostatic latent image formed on a cylindrical photosensitive body constituting an image carrier. Further, there is provided an image transferring device that transfers the visible image onto a paper as a recording medium from the cylindrical photosensitive body. Most of the toner on the photosensitive body is transferred onto the paper by this transfer device, however some of the toner is left behind on the photosensitive body.
As shown in FIG. 1, this toner may be left behind on the photosensitive body after image transfer in various ways. A shows the normal transfer process. B is similar to the normal transfer process, however, as a result of somewhat excessive transfer, the toner is given a charge of opposite polarity i.e. a positive charge, produced by the transfer charger. This is likely to occur when solid black areas etc. are transferred. C is a case where poor image transfer leaves behind lumps of toner. This is likely to occur when the paper is separated from the photosensitive body due to carrier adhesion from paper creasing during transfer. The toner remaining in unexposed areas after transfer is electrostatically attracted to the developing device since the surface potential V.sub.0 of the photosensitive body is lower than the developing bias Vb that is applied to the developing device. In contrast, toner left behind in the exposed areas after transfer, being negatively charged toner, does not return to the developing device but is left behind on the photosensitive body, since the surface potential of the photosensitve body is increased by exposure. A method of cleaning the photosensitive body by electrostatically attracting the remained toner from the photosensitive body to the developing device is thoroughly described in U.S. Pat. No. 4,769,676.
If residual toner is left behind on the photosensitive body after transfer as in cases A and B, where the toner is uniformly dispersed over the photosensitive body, the residual toner does not effect the next image formation. Because the amount of light provided by the laser in the exposing step is large and the exposed area is large in relation to the size of the toner, the exposure is not obstructed by such toner adhering to the photosensitive body. Therefore, the latent image can be formed by decay of the negative potential of the surface of the photosensitive body. Furthermore, there is no difficulty at all in the formation of the latent image, due to the effective cleaning by the developing device, as described above, in the unexposed areas. However, in case C, where lumps of toner are left behind, the developing device has difficulty in cleaning away all this toner, and "positive memories" are formed on white backgrounds. And in cases where these portions correspond to subsequently exposed areas, the toner layer becomes a thick lump of large diameter, preventing the passage of light and resulting in negative memories on black backgrounds.
In order to remove such toner left behind after transfer as described above, conventionally, a cleaner was provided at the periphery of the photosensitive body, between the transfer charger and the discharge lamp. This cleaner conventionally consisted of a blade that contacted the surface of the photosensitive body to scrape off residual toner, and a recovery box in which this scraped-off toner was recovered.
However, with the conventional cleaner as described above, since it employs a construction wherein the residual toner left behind on the photosensitive body was scraped away by a blade into a recovery box, carrying out 2,000 to 3,000 copies would normally result in the recovery box being filled with toner, and so becoming unusable. For this reason, complicated recovery mechanisms and recovery boxes were provided and the toner had to be recovered by changing the recovery box each time. However, these mechanisms were of complicated construction, and operability was poor, in that they soiled the hands or clothes of the user. In some machines, the cleaner had to be thrown away every time the recovery box became full. Furthermore, due to use of a blade to scrape away the toner from the photosensitive body, the surface of the photosensitive body was damaged. A further problem was that, if paper got caught between the blade and the surface of the photosensitive body, proper feeding of the paper was prevented.
U.S. Pat. No. 4,769,676 discloses a dispersing device for dispersing the residual toner remaining on a photosensitive drum after the transfer of the developed image and before the next image forming operation. The dispersing device is composed with a conductive brush which is in sliding contact with the photosensitive drum. By sliding contact conductive brush with photosensitive drum, the strewing of residual toner is effected.
However, since the dispersing device disclosed in U.S. Pat. No. 4,769,676 is composed with only one conductive brush, a toner retaining capability on the conductive brush is small. Therefore, although the residual toner is attracted to the conductive brush, since the residual toner is separated at once from the conductive brush, a toner dispersing capability is small. As a result, the residual toner is unfortunately transferred on the next recording medium in the next image forming operation.